If you need to find a string in a MySQL database (searching across all available tables), theres a couple of options for achieving this; the easiest is if you have phpMyAdmin installed in which case you can use the built in search functionality. If this isn't something you've got installed on your server the alternative is to download the file and search in your favourite editor.
PHPMyAdmin has a built in search feature that allows you to search multiple tables at the same time. To use it, login to PHPMyAdmin and go to the 'Search' tab. Enter the search term in the first field and set the critera type. If you want to search all tables and fields that may contain 'hello' then use a wildcard like so:
You can also restrict the search to specific columns using the last field.
An alternative that may be useful depending on how you want to manipulate the data afterwards is to export the database and then you can perform standard string search searches in an editor.
To export a database via PHPMyAdmin use the export tab and click go with the default options. To do it via command line you can use the following:
mysqldump -u yourusername -p yourpassword yourdatabasename > outputfile.sql